Optical surface plasmon resonance (SPR) sensors are sensitive to changes in the refractive index (RI) of a sample near the sensor surface. Most bulk prism SPR sensor configurations measure either the angular reflection spectrum for monochromatic light or the wavelength reflection spectrum for collimated white light.
Various SPR sensor configurations utilizing waveguides, including optical fibers, have been reported. These include a single-mode planar waveguiding structure which detects intensity changes in monochromatic light (Lavers, C. R. and Wilkinson, J. S. (1994) "A Waveguide-Coupled Surface-Plasmon Sensor for an Aqueous Environment" Sensors and Actuators B 22:75-81) and a sensor system in which white light is injected into a single-mode waveguide having an SPR supporting superstructure (Kreuwel, H. J. M. et al. (1987) "Surface Plasmon Dispersion and Luminescence Quenching Applied to Planar Waveguide Sensors for the Measurement of Chemical Concentrations," Proc. SPIE 798:218-224; Lambeck, P. V. (1992) "Integrated Opto-Chemical Sensors" Sensors and Actuators B 8:103-116). Additionally, a white light multi-mode fiber optic SPR sensor has been introduced (U.S. Pat. No. 5,359,681, issued October 1994); Jorgenson, R. C. and Yee, S. S. (1993) "A Fiber Optic Chemical Sensor Based on Surface Plasmon Resonance," Sensors and Actuators B 12:213; Jung, C. C. et al. (1995) "Fiber-Optic Surface Plasmon Dispersive Index Sensor for Highly Opaque Samples" Process Control and Quality 7:167-171; Jorgenson, R. C. and Yee, S. S. (1994) "Control of the Dynamic Range and Sensitivity of a Surface Plasmon Resonance Based Fiber Optic Sensor" Sensors and Actuators A 43:4448; Mar, M. et al. (1993 ) "In-Situ Characterization of Multilayered Langmuir-Blodgett Films Using a Surface Plasmon Resonance Fiber Optic Sensor" Proc. of the 15th Annual Conf. of the IEEE Engineering in Medicine and Biology Soc., San Diego, Calif. pp. 1551-1552.
U.S. Pat. No. 5,485,277 (filed Jul. 26, 1994, issued Jan. 16, 1996) "Surface Plasmon Resonance Sensor and Methods for the Utilization Thereof " reports an SPR sensor said to comprise a "waveguide" cartridge, a cylindrical diverging lens coupled to the "waveguide" and a plurality of photodetectors optically connected to the cylindrical lens. The "waveguide", exemplified by a microscope slide with angled input and output faces, carries a symmetrically positioned metal layer that supports SPR. Apparently, monochromatic light is introduced into the "waveguide" through the angled polished input face by focusing the light through the end of the "waveguide" onto the metal sensor surface at a range of angles spanning angular location of the surface plasmon resonance. The angle of the input face of the "waveguide" is polished to an angle .alpha..sub.wvg, where ##EQU1## and n.sub.wvg is the refractive index of the "waveguide" and .theta..sub.central is the center angle of the range of rays directed at the sensing surface. It appears that .theta..sub.central is very close to .theta..sub.SPR, the surface plasmon resonance angle. The RI of the sample is determined by measuring light intensity exiting the sensor as a function of angle. The patent also discusses the use of sensing and reference channels on the same metal film-coated waveguide.
SPR sensor waveguide configurations that have been reported are limited to those measuring refractive index at either a single wavelength or a single angle (angular or wavelength modulation, respectively). Such configurations are considered zero order sensors since they measure only one independent variable for a given analyte. Recently, a first order SPR sensor geometry that can simultaneously measure a sample's index of refraction at multiple wavelengths was reported (Karlsen, S. R. et al. (1995), "Simultaneous Determination of Refractive Index and Absorbance Spectra of Chemical Samples Using Surface Plasmon Resonance," Sensors and Actuators B 24-25:747-749). This dispersive RI sensor which employed a cylindrical sapphire prism with a gold sensing layer required several discrete optical components which introduced optical aberrations in the reflected signal, making it difficult to calibrate both the angular and spectral outputs of the sensor.
Thus there remains a need in the art for first order SPR sensors that allow independent measurements of two variables for a given analyte providing dispersive RI information and for SPR sensors allowing the simultaneous determination of two parameters, for example, film thickness and RI of a thin film applied to an SPR sensing surface. There is also a need in the art, particularly for assay of biological samples, for SPR sensors that can simultaneously detect more than one analyte in a sample (multiplexed sensors). There also generally remains a need for SPR sensors which are sensitive, simple to use, readily calibrated, compact in size and rugged, and inexpensive to produce. SPR sensors of this invention meet these needs.